Re: [Xen-devel] [PATCH v2 5/5] x86: Use alternative mechanism to define CLAC/STAC

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/05/2014 06:34, Feng Wu wrote:
> This patch use alternative mechanism to define CLAC/STAC.
>
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper@xxxxxxxxxx>

> ---
>   xen/include/asm-x86/asm_defns.h | 24 +++++++++++++-----------
>   1 file changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
> index 0b9f898..dd50b97 100644
> --- a/xen/include/asm-x86/asm_defns.h
> +++ b/xen/include/asm-x86/asm_defns.h
> @@ -9,6 +9,7 @@
>   #include <asm/processor.h>
>   #include <asm/percpu.h>
>   #include <asm/cpufeature.h>
> +#include <asm/alternative.h>
>   
>   #ifndef __ASSEMBLY__
>   void ret_from_intr(void);
> @@ -165,26 +166,27 @@ void ret_from_intr(void);
>   #define __ASM_STAC      .byte 0x0f,0x01,0xcb
>   
>   #ifdef __ASSEMBLY__
> -#define ASM_AC(op)                                       \
> -        btl $X86_FEATURE_SMAP & 31,                      \
> -        CPUINFO_FEATURE_OFFSET(X86_FEATURE_SMAP)+boot_cpu_data(%rip); \
> -        jnc 881f;                                        \
> -        __ASM_##op;                                      \
> -881:
> -
> +#define ASM_AC(op)                                                     \
> +        661: ASM_NOP3;                                                 \
> +        .pushsection .altinstr_replacement, "ax";                      \
> +        662: __ASM_##op;                                               \
> +        .popsection;                                                   \
> +        .pushsection .altinstructions, "a";                            \
> +        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;       \
> +        .popsection
>   #define ASM_STAC ASM_AC(STAC)
>   #define ASM_CLAC ASM_AC(CLAC)
>   #else
>   static inline void clac(void)
>   {
> -    if ( boot_cpu_has(X86_FEATURE_SMAP) )
> -        asm volatile (STR(__ASM_CLAC) : : : "memory");
> +    /* Note: a barrier is implicit in alternative() */
> +    alternative(ASM_NOP3, STR(__ASM_CLAC), X86_FEATURE_SMAP);
>   }
>   
>   static inline void stac(void)
>   {
> -    if ( boot_cpu_has(X86_FEATURE_SMAP) )
> -        asm volatile (STR(__ASM_STAC) : : : "memory");
> +    /* Note: a barrier is implicit in alternative() */
> +    alternative(ASM_NOP3, STR(__ASM_STAC), X86_FEATURE_SMAP);
>   }
>   #endif
>   

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